- ID:
- ivo://CDS.VizieR/J/ApJ/890/132
- Title:
- MgII abs. toward SDSS QSOs with rotation measures
- Short Name:
- J/ApJ/890/132
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We probed the magnetic fields in high-redshift galaxies using excess extragalactic contribution to residual rotation measure (RRM) for quasar sightlines with intervening MgII absorbers. Based on a large sample of 1132 quasars, we have computed RRM distributions broadening using median absolute deviation from the mean ({sigma}_rrm_^md^), and found it to be 17.1+/-0.7rad/m^2^ for 352 sightlines having MgII intervening absorbers in comparison to its value of 15.1+/-0.6rad/m^2^ for 780 sightlines without such absorbers, resulting in an excess broadening ({sigma}_rrm_^ex^) of 8.0+/-1.9rad:m^2^ among these two subsamples. This value of {sigma}_rrm_^ex^, has allowed us to constrain the average strength of magnetic field (rest frame) in high-redshift galaxies responsible for these Mg II absorbers, to be ~1.3+/-0.3{mu}G at a median redshift of 0.92. This estimate of magnetic field is consistent with the reported estimate in earlier studies based on radio-infrared correlation and energy equipartition for galaxies in the local universe. A similar analysis on subsample split based on the radio spectral index, {alpha} (with F_{nu}_{propto}{nu}^{alpha}^), for flat ({alpha}>=-0.3; 315 sources) and steep ({alpha}<=-0.7; 476 sources) spectrum sources shows a significant {sigma}_rrm_^ex^ (at 3.5{sigma} level) for the former and absent in the latter. An anticorrelation found between the {sigma}_rrm_^md^ and percentage polarization (p) with a similar Pearson correlation of -0.62 and -0.87 for subsamples with and without MgII, respectively, suggests the main contribution for decrements in the p value to be intrinsic to the local environment of quasars.
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- ID:
- ivo://CDS.VizieR/J/ApJS/151/271
- Title:
- Millennium Arecibo 21-cm Survey. III.
- Short Name:
- J/ApJS/151/271
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We outline the theory and practice of measuring the four Stokes parameters of spectral lines in emission/absorption observations. We apply these concepts to our Arecibo H I absorption line data and present the results. We include a detailed discussion of instrumental effects arising from polarized beam structure and its interaction with the spatially extended emission line structure. At Arecibo, linear polarization [Stokes (Q,U)] has much larger instrumental effects than circular (Stokes V). We show how to reduce the instrumental contributions to V and to evaluate upper limits to its remaining instrumental errors by using the (Q,U) results. These efforts work well for opacity spectra but not for emission spectra. Arecibo's large central blockage exacerbates these effects, particularly for emission profiles, and other telescopes with weaker sidelobes are not as susceptible. We present graphical results for 41 sources; we analyze these absorption spectra in terms of Gaussian components, which number 136, and present physical parameters including magnetic field for each.
- ID:
- ivo://CDS.VizieR/J/ApJS/213/13
- Title:
- 1.3mm polarization maps of star-forming cores & SFRs
- Short Name:
- J/ApJS/213/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present {lambda}1.3mm Combined Array for Research in Millimeter-wave Astronomy observations of dust polarization toward 30 star-forming cores and eight star-forming regions from the TADPOL survey. We show maps of all sources, and compare the ~2.5" resolution TADPOL maps with ~20" resolution polarization maps from single-dish submillimeter telescopes. Here we do not attempt to interpret the detailed B-field morphology of each object. Rather, we use average B-field orientations to derive conclusions in a statistical sense from the ensemble of sources, bearing in mind that these average orientations can be quite uncertain. We discuss three main findings. (1) A subset of the sources have consistent magnetic field (B-field) orientations between large (~20") and small (~2.5") scales. Those same sources also tend to have higher fractional polarizations than the sources with inconsistent large-to-small-scale fields. We interpret this to mean that in at least some cases B-fields play a role in regulating the infall of material all the way down to the ~1000AU scales of protostellar envelopes. (2) Outflows appear to be randomly aligned with B-fields; although, in sources with low polarization fractions there is a hint that outflows are preferentially perpendicular to small-scale B-fields, which suggests that in these sources the fields have been wrapped up by envelope rotation. (3) Finally, even at ~2.5" resolution we see the so-called polarization hole effect, where the fractional polarization drops significantly near the total intensity peak.
- ID:
- ivo://CDS.VizieR/J/AJ/149/60
- Title:
- Modeling Faraday structures. I. 1100-1400 MHz
- Short Name:
- J/AJ/149/60
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Faraday rotation measures (RMs) and more general Faraday structures are key parameters for studying cosmic magnetism and are also sensitive probes of faint ionized thermal gas. A definition of which derived quantities are required for various scientific studies is needed, as well as addressing the challenges in determining Faraday structures. A wide variety of algorithms has been proposed to reconstruct these structures. In preparation for the Polarization Sky Survey of the Universe's Magnetism (POSSUM) to be conducted with the Australian Square Kilometre Array Pathfinder and the ongoing Galactic Arecibo L-band Feeds Array Continuum Transit Survey (GALFACTS), we run a Faraday structure determination data challenge to benchmark the currently available algorithms, including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling, and QU-fitting. The input models include sources with one Faraday thin component, two Faraday thin components, and one Faraday thick component. The frequency set is similar to POSSUM/GALFACTS with a 300MHz bandwidth from 1.1 to 1.4GHz. We define three figures of merit motivated by the underlying science: (1) an average RM weighted by polarized intensity, RM_wtd_, (2) the separation {Delta}_{phi}_ of two Faraday components, and (3) the reduced chi-squared {Chi}_r_^2^. Based on the current test data with a signal-to-noise ratio of about 32, we find the following. (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found. (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RM_wtd_ but with significantly higher errors for {Delta}_{phi}_. All other methods, including standard Faraday synthesis, frequently identify only one component when {Delta}_{phi}_ is below or near the width of the Faraday point-spread function. (3) No methods as currently implemented work well for Faraday thick components due to the narrow bandwidth. (4) There exist combinations of two Faraday components that produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model. Further exploration of all these issues is required before upcoming surveys will be able to provide reliable results on Faraday structures.
- ID:
- ivo://CDS.VizieR/J/AJ/144/105
- Title:
- MOJAVE. VIII. Faraday rotation in AGN jets.
- Short Name:
- J/AJ/144/105
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report observations of Faraday rotation measures for a sample of 191 extragalactic radio jets observed within the MOJAVE program. Multifrequency Very Long Baseline Array observations were carried out over 12 epochs in 2006 at four frequencies between 8 and 15GHz. We detect parsec-scale Faraday rotation measures in 149 sources and find the quasars to have larger rotation measures on average than BL Lac objects. The median core rotation measures are significantly higher than in the jet components. This is especially true for quasars where we detect a significant negative correlation between the magnitude of the rotation measure and the de-projected distance from the core. We perform detailed simulations of the observational errors of total intensity, polarization, and Faraday rotation, and concentrate on the errors of transverse Faraday rotation measure gradients in unresolved jets. Our simulations show that the finite image restoring beam size has a significant effect on the observed rotation measure gradients, and spurious gradients can occur due to noise in the data if the jet is less than two beams wide in polarization. We detect significant transverse rotation measure gradients in four sources (0923+392, 1226+023, 2230+114, and 2251+158). In 1226+023 the rotation measure is for the first time seen to change sign from positive to negative over the transverse cuts, which supports the presence of a helical magnetic field in the jet. In this source we also detect variations in the jet rotation measure over a timescale of three months, which are difficult to explain with external Faraday screens and suggest internal Faraday rotation. By comparing fractional polarization changes in jet components between the four frequency bands to depolarization models, we find that an external purely random Faraday screen viewed through only a few lines of sight can explain most of our polarization observations, but in some sources, such as 1226+023 and 2251+158, internal Faraday rotation is needed.
- ID:
- ivo://CDS.VizieR/J/ApJ/862/151
- Title:
- MOJAVE. XVI: parsec-scale AGN jet cores at 15GHz
- Short Name:
- J/ApJ/862/151
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an analysis of the core linear polarization properties of 387 parsec-scale active galactic nuclei (AGNs) jets. Using 15GHz VLBA data, we revisit the conclusions of the first paper in this series with multiepoch measurements and more detailed analysis of a larger AGN sample that spans a broader range of synchrotron peak frequencies. Each AGN has been observed for at least five epochs between 1996 and 2017. We find that BL Lac objects have core electric vector position angles (EVPAs) that tend toward alignment with the local jet direction; compared to flat spectrum radio quasars, their EVPAs are also less variable over time. The AGN cores that are most fractionally polarized and least variable in polarization have EVPAs that are closely aligned with the local jet direction; they also have low variability in EVPA. These results support the popular model of a standing transverse shock at the base of the jet that collimates the jet magnetic field perpendicular to the jet direction, increasing the fractional polarization and leading to greater polarization stability over time. High-synchrotron-peaked BL Lac objects form a low luminosity, low fractional polarization population. The five narrow line Seyfert 1 galaxies in our sample have low fractional polarization and large EVPA-jet misalignments. Although AGNs detected at {gamma}-rays are thought to be more Doppler boosted than nondetected AGNs, we find no significant differences in fractional polarization based on detection by Fermi-LAT; the {gamma}-loud AGNs are, however, more variable in core EVPAs.
- ID:
- ivo://CDS.VizieR/J/MNRAS/381/341
- Title:
- MRCR-SUMSS Ultra-steep-spectrum (USS) sample
- Short Name:
- J/MNRAS/381/341
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This paper introduces a new program to find high-redshift radio galaxies in the Southern hemisphere through ultra-steep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices {alpha}^843^_408_<=-1.0 (S_{nu}_{prop.to}{nu}^alpha^) and flux densities S_408_>=200mJy in a region of 0.35sr, chosen by cross-correlating the revised 408MHz Molonglo Reference Catalogue, the 843MHz Sydney University Molonglo Sky Survey and the 1400MHz NRAO VLA Sky Survey in the overlap region -40{deg}<DE<-30{deg}. We present Australia Telescope Compact Array (ATCA) high-resolution 1384 and 2368MHz radio data for each source, which we use to analyse the morphological, spectral index and polarization properties of our sample. We find that 85 per cent of the sources have observed-frame spectral energy distributions that are straight over the frequency range 408-2368MHz, and that, on average, sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization. Fractional polarization is anticorrelated with flux density at both 1400 and 2368MHz. We also use the ATCA data to determine observed-frame Faraday rotation measures for half of the sample.
148. M 31 Stokes images
- ID:
- ivo://CDS.VizieR/J/A+A/633/A5
- Title:
- M 31 Stokes images
- Short Name:
- J/A+A/633/A5
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetic fields play an important role in the dynamics and evolution of galaxies; however, the amplification and ordering of the initial seed fields are not fully understood. The nearby spiral galaxy M 31 is an ideal laboratory for extensive studies of magnetic fields. Our aim was to measure the intrinsic structure of the magnetic fields in M 31 and compare them with dynamo models of field amplification. The intensity of polarized synchrotron emission and its orientation are used to measure the orientations of the magnetic field components in the plane of the sky. The Faraday rotation measure gives information about the field components along the line of sight. With the Effelsberg 100-m telescope three deep radio continuum surveys of the Andromeda galaxy, M 31, were performed at 2.645, 4.85, and 8.35 GHz (wavelengths of 11.3, 6.2, and 3.6 cm). The {lambda} 3.6 cm survey is the first radio survey of M 31 at such small wavelengths. Maps of the Faraday rotation measures (RMs) are calculated from the distributions of the polarization angle. At all wavelengths the total and polarized emission is concentrated in a ring-like structure of about 7-13 kpc in radius from the centre. Propagation of cosmic rays away from the star-forming regions is evident. The ring of synchrotron emission is wider than the ring of the thermal radio emission, and the radial scale length of synchrotron emission is larger than that of thermal emission. The polarized intensity from the ring in the plane of the sky varies double-periodically with azimuthal angle, indicating that the ordered magnetic field is oriented almost along the ring, with a pitch angle of -14{deg}+/-2{deg} at {lambda}6.2 cm. The RM varies systematically along the ring. The analysis shows a large-scale sinusoidal variation with azimuthal angle, signature of an axisymmetric spiral (ASS) regular magnetic field, plus a superimposed double-periodic variation of a bisymmetric spiral (BSS) regular field with about six times smaller amplitude. The RM amplitude of (118+/-3) rad/m^2^ between {lambda}6.2 cm and {lambda}3.6 cm is about 50% larger than between {lambda}11.3 cm and {lambda}6.2 cm, indicating that Faraday depolarization at {lambda}11.3 cm is stronger (i.e. with a larger Faraday thickness) than at {lambda}6.2 cm and {lambda}3.6 cm. The phase of the sinusoidal RM variation of -7{deg}+/-1{deg} is interpreted as the average spiral pitch angle of the regular field. The average pitch angle of the ordered field, as derived from the intrinsic orientation of the polarized emission (corrected for Faraday rotation), is significantly smaller: -26{deg}+/-3{deg}. The dominating ASS plus the weaker BSS field of M 31 is the most compelling case so far of a field generated by the action of a mean-field dynamo. The difference in pitch angle of the regular and the ordered fields indicates that the ordered field contains a significant fraction of an anisotropic turbulent field that has a different pattern than the regular (ASS + BSS) magnetic field.
- ID:
- ivo://CDS.VizieR/J/PASP/106/949
- Title:
- Multicolor polarimetry of Be stars
- Short Name:
- J/PASP/106/949
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Optical polarization data in the UBVRI filter system for eight bright northern Be stars are presented here as the continuation of a long-term monitoring project begun in 1984. There are no strong cases of night-to-night variability, and the only star showing unmistakable changes in polarization from year to year over the nine years covered by the program is Pi Aquarii. Even though the observed sample spans a wide range in spectral type, vsini, and degree of intrinsic polarization, the normalized wavelength dependence of the polarization is surprisingly similar for all of the stars. Analysis of the wavelength dependence of the variable polarization of {pi} Aqr in terms of a simple equatorial-disk model suggests that changes in the circumstellar electron number density alone may be sufficient to account for the observations, but it is not clear what real physical mechanism is involved.
- ID:
- ivo://CDS.VizieR/J/MNRAS/301/235
- Title:
- Multifrequency polarimetry of 300 pulsars
- Short Name:
- J/MNRAS/301/235
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Polarimetric observations of 300 pulsars have been conducted with the 76-m Lovell telescope at Jodrell Bank at radio frequencies centred around 230, 400, 600, 920, 1400 and 1600MHz. More than half of the pulsars have no previously published polarization profiles and this compilation represents about three times the sum of all previously published pulsar polarization data. A selection of integrated polarization profiles is provided. Tables of pulse widths and the degree of both linear and circular polarization are given for all pulsars, and these act as an index for all the data, which are available by anonymous ftp in numerical and graphical form.
